Methods for generating microdroplets of a uniform volume at a regular frequency are well known in the art. However, sample to sample variations in viscosity, viscoelasticity, surface tension or other physical properties of the sample fluid coming from, but not limited to, the inclusion of polymers, detergents, proteins, cells, nucleic acids or buffering solutions, influence the droplet size and volume and, hence, the frequency of generation in an unpredictable way. Thus, the same nozzle on the same microfluidic substrate with same carrier fluid, but a different dispersed fluid will result in a different droplet volume at a different frequency. These limitations also have an impact on the extent to which volumes can be reproducibly combined. Together with typical variations in pump flow rate precision and variations in channel dimensions, microfluidic systems are severely limited without a means to compensate on a run-to-run basis.
As a result of the above factors, current microdroplet technologies cannot efficiently or reliably be used for applications involving combining droplets of different species at high frequencies. Consequently, there is a need in the art for methods of precise control, manipulation and regulation of droplet frequency generation, frequency of library droplet introduction and droplet volume.